XPost: sci.space.policy   
   From: henry@spsystems.net   
      
   In article ,   
   Earl Colby Pottinger   wrote:   
   >> People are still struggling to make imaging interferometry work well at   
   >> distances of a hundred *meters* with both telescopes resting on solid rock.   
   >   
   >Henry, correct me if I am wrong.  But I thought with present day computers   
   >you don't need to hold the distance steady between the scopes.  Rather you   
   >needed to know the distance to a fraction of a wavelenght between the scopes   
   >at the time the signals are recorded.   
      
   The key requirement is that you need to be able to measure the relative   
   phase of the photons arriving at the two scopes.  There are two ways you   
   can do that, in principle.   
      
   One way is to bring those photons together so they interfere, and measure   
   the results of the interference.  That gives you a direct readout of the   
   relative phase.  The length of the optical path that brings the photons   
   together must be stable -- or at least continuously measurable -- to a   
   fraction of a wavelength, because any change in its length adds spurious   
   phase differences between the photons.   
      
   The other thing you can do is *record* the phases of the photons at each   
   scope, along with very precise time references, so you can compare them   
   later at your leisure, essentially simulating the interference in the   
   computer.  The problem is, we have no way to do that for *light*.  The   
   required speed of the phase measurement and recording system is four or   
   five orders of magnitude higher than that for radio.   
      
   >Opps, just realized with long radiowaves even the phase shift can be measured   
   >- if you need the phase of visible light to be recorded at both scopes at the   
   >same time I don't think we have reach that tech yet.   
      
   Exactly.   
   --   
   "Think outside the box -- the box isn't our friend."    |   Henry Spencer   
                                   -- George Herbert       | henry@spsystems.net   
      
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